The efficient manipulation of subsea structures and devices, such as wellheads, may require an interface between a control or processing facility, such as a platform, a land base facility, or a servicing vessel with a subsea facility, such as a wellhead or a ROV. It is frequently desirable to be able to remotely manipulate valves and other equipment as well as inject various servicing fluids into the subsea facility and to transmit and receive signals and electrical power. Umbilicals may be used to provide this necessary interface.
Many conventional umbilicals incorporate a plurality of internal steel tubes shrouded by an external plastic jacket. The steel tubes may be used to carry hydraulic fluids, hydration inhibitors, such as methanol, or other servicing fluids. In some cases, electrical conductors may be positioned around or within the steel tubes.
Steel as an internal tube material may present one or more disadvantages associated with fatigue strength limitations, bending capacity, weight and/or cost. Shear weight is another disadvantage associated with steel internal tubing in umbilicals. As the weight of the umbilical goes up, more of the total stress capacity of the umbilical must be devoted to tension and thus less is left over for bending stresses. In addition, heavier umbilicals require more robust handling equipment, such as winches, spools and the structures holding them, such as servicing vessels and/or platforms.
Material cost may be another drawback associated with conventional steel umbilicals. Most conventional steel umbilical tubing is fabricated from sophisticated alloys that require complex hot and cold working. As many types of umbilicals may be thousands of feet in length, material costs due to the steel can be large.
U.S. Pat. No. 6,472,614 discloses in an umbilical, one or more steel rods, which provide strength and ballast, are wound helically within the umbilical along with the steel tubes and/or elongated active umbilical elements. These steel rods replace some or all of the thermoplastic filler elements that would otherwise be included within the umbilical. An umbilical may include a plurality of steel tubes helically wound around a core, and at least one substantially solid steel rod helically wound around the core, the steel rod being arranged in a void between the steel tubes. U.S. Pat. No. 6,472,614 is herein incorporated by reference in its entirety.
As the length of the umbilical increases, the weight also increases. At a given length for an umbilical, the steel tubes are no longer able to support the axial stress due to the weight of the umbilical. Making the steel tubes larger or thicker would not be a solution to the depth problem, as larger or thicker steel tubes would be stronger but also heavier, and for a given length, the umbilical would not be able to support its weight. Similarly, adding steel rods would add to the strength, but also the weight of the umbilical, which for a given length for the umbilical would not able to support its weight.
There is a need in the art for umbilicals that have sufficient strength to be used in long length applications.